Tips: Take care of your folds at the beginning, and the rest should be fairly easy if you follow the tutorial exactly. Making careful folds, and making sure everything is properly aligned will allow you to get best results for the final product. This rule is something that should probably be kept in mind when making any origami model. Other than that, have fun!

More about the Minecraft Slime:

A slime is a hostile, animate, green, gelatinous, hopping mob in the shape of a cube that appears in various sizes. They are able to see the player through solid blocks. When killed, a slime will split into smaller slimes, except for the smallest slime which will drop slimeballs instead. Slimes are found in either the lowest 40 layers of certain chunks, or (as of the Pretty Scary Update) in swamp biomes in darkness. On Xbox 360, slimes can also be found below level 40 since TU8(below 16 before that). Slimes can spawn in superflat world types if its world generation level is below level 40.